A "local terminal" is defined herein to mean a communication device which is the initiator of an attempt to establish a digital communication channel with another communication device. A "remote terminal" is defined herein to mean a communication device which is the responder to an attempt by the local terminal to establish a digital communication channel.
For example, a secure telephone unit (STU) is capable of establishing a "secure" communication channel with another STU. Secure communications comprises encrypting, transmitting, receiving and decrypting digital data. The modem training procedure begins when one STU "initiates" the establishment of the secure communication channel (e.g., the user of one STU presses the "secure" button) .
FIG. 1 depicts an inoperative configuration of conventional communication system 101. Communication system 101 comprises local terminal 110, modems 114, 138, radio units 118, 132, 134, communication satellites 130, public switched telephone network 141 (PSTN), analog links 112, 140, 150, RF digital links 120, digital links 116, 136 and remote terminal 152. PSTN 141 comprises, for example, communication satellites 131 and terrestrial telephone networks 142, 146, 148 (TTNs). TTNs 142, 146, 148 communicate with communication satellites 131 via RF analog links 144. TTNs 142, 146, 148 may alternatively be inter-connected via wirelines (not illustrated in FIG. 1). Links 112, 116, 136, 140 and 150 are wireline links and links 120 and 144 are RF satellite links.
Local terminal 110 and remote terminal 152 produce digital bitstreams modulated by internal modems (not illustrated in FIG. 1) to produce modulated carriers that may be transmitted via analog links 112 and 150, respectively. For example, when local terminal 110 and remote terminal 152 are STUs, their internal modems may produce encrypted modulated carriers. Modulated carriers received via analog links 112 and 150 are demodulated by the internal modems of local terminal 110 and remote terminal 152 to produce digital bitstreams that may be processed by terminals 110, 152.
Modems 114, 138 modulate incoming digital bitstreams and demodulate incoming modulated carriers. As a result, signals communicated via analog links 112, 140, 150 and RF analog links 144 are modulated carriers and signals communicated via RF digital links 120 and digital links 116, 136 are digital bitstreams.
Satellites 130 are narrow-band digital satellites. Signals communicated via RF digital links 120 may be limited to bandwidths as low as 2400 bits per second (bps). Satellites 131 are wide-band satellites having bandwidths of 32 per second (kbps) or 64 kbps, for example. Signals communicated via RF analog links 144 comprise trunked channels from TTNs 142, 146, 148.
For example, local terminal 110, analog link 112, modem 114, digital link 116 and radio units 118, 132 may be located off shore. Radio unit 134, digital link 136, modem 138, analog links 140, 150, PSTN 141 and remote terminal 152 may be land-based equipment.
A "satellite hop" is defined herein to mean a communication path from a system node (e.g., radio unit 118, TTN 142, etc.) up to a satellite and down to another system node or other communication apparatus. Multiple communication satellites 130 and system nodes may exist between local terminal 110 and land-based modem 138. Additional communication satellites 131 may exist between land-based modem 138 and remote terminal 152. FIG. 1 depicts a total of four satellite hops between local terminal 110 and remote terminal 152.
To establish a digital communication link within conventional communication system 101, the internal modem of local terminal 110 must "train" with the internal modem of remote terminal 152 to adaptively equalize the line and set near and far echo taps for echo cancellation. This is all performed digitally within the internal modems as part of the modem training procedure. Near the beginning of the modem training procedure, messages describing modem capabilities may be exchanged between internal modems so that the internal modems may determine a desired data rate, among other things. For some modes of operation, capabilities messages need not be exchanged.
FIG. 2 represents the timing of modem training messages exchanged between local terminal 110 and remote terminal 152 for an operative configuration of conventional communication system 101. As used in FIG. 2, "tx" is an abbreviation for "transmit" and "rx" is an abbreviation for "receive".
Referring also to FIG. 1 and associated text, the modem training procedure is initiated by local terminal 10. Modems 114 and 138 increase the propagation time of the modem training signals through communication system 101. For example, modem 114 requires approximately 0.5 seconds to detect local modem tone 210 (referred to also as LMT). Other than adding delay, modems 114 and 138 are transparent during the modem training procedure between local terminal 110 and remote terminal 152.
Local terminal 110 transmits local modem tone 210 to remote terminal 152 beginning at time 240. For example, local modem tone 210 may be a 2100 Hz tone of limited duration. Local terminal 110 continues transmission of local modem tone 210 until local terminal 110 begins reception of remote modem tone 215 (referred to also as RMT).
Remote terminal 152 receives local modem tone 210 beginning at time 245. Remote terminal 152 may then wait a certain signaling delay time and transmit remote modem tone 215 to local terminal 110 beginning at time 250. For example, remote modem tone 215 may be a P1800 Hz tone of limited duration. A P1800 (or "Pseudo" 1800) Hz tone consists of alternations of dibits 00 and 10, corresponding to +45 degree and -45 degree phase shifts, respectively.
Local terminal 110 receives remote modem tone 215 beginning at time 255. First response time-out interval 260, monitored by local terminal 110, begins at time 240 when local terminal 110 starts transmitting local modem tone 210. Local terminal 110 "fails the call" (e.g., hangs up) if it does not begin receiving remote modem tone 215 within first response time-out interval 260. Alternatively, local terminal 110 may re-initiate the modem training procedure to attempt to establish communications with remote terminal 152.
Capabilities messages 220, 225 are exchanged by local terminal 110 and remote terminal 152 indicating the "capabilities" of each terminal's internal modem. The exchanged capabilities messages 220, 225 are interpreted according to a predetermined hierarchy to arrive at negotiated parameters (e.g., data rate, etc.) which determine how further communications will be handled. Capabilities messages 220, 225 contain information the terminals use to select a common mode of operation (e.g., a negotiated data rate of 4800 bits per second).
Local terminal 110 transmits local capabilities message 220 (referred to also as LCM) beginning at time 275. Remote terminal 152 receives local capabilities message 220 beginning at time 280.
Remote terminal 152 transmits remote capabilities message 225 (referred to also as RCM) beginning at time 285. Remote capabilities message 225 is received by local terminal 110 beginning at time 290. Second response time-out interval 270, monitored by local terminal 110, begins at time 275, when local terminal 110 starts transmitting local capabilities message 220. Local terminal 110 fails the call if it does not begin receiving remote capabilities message 225 within second response time-out interval 270. Alternatively, local terminal 110 may re-initiate the modem training procedure to attempt to establish communications with remote terminal 152.
FIG. 3 is a flow diagram of a prior art protocol for local terminal modem training and capabilities message exchange. Referring also to FIGS. 1 and 2 and associated text, local terminal modem training and capabilities message exchange begins (block 310) when local terminal 110 transmits local modem tone 210 (block 315). Local terminal 110 starts an internal timer (block 320) when it begins transmission of local modem tone 210. Local terminal 110 then determines whether the internal timer value exceeds first response time-out interval 260 (block 325). When the internal timer value exceeds first response time-out interval 260 (block 325), local terminal 110 assumes remote terminal 152 is nonexistent or incapable of establishing communications and local terminal 110 fails the call (block 355), thus terminating the modem training procedure. For example, 3.3 +/-0.7 seconds is a standard first response time-out interval within the telecommunications industry.
When the internal timer value does not exceed first response time-out interval 260 (block 325), local terminal 110 determines whether remote modem tone 215 has been received (block 330). When remote modem tone 215 has not been received (block 330), local terminal 110 again determines whether first response time-out interval 260 has been exceeded (block 325). The procedure then iterates as shown in FIG. 3.
When remote modem tone 215 has been received (block 330), local terminal 110 transmits local capabilities message 220 (block 335). Local terminal 110 starts an internal timer (block 340) when it begins transmission of local capabilities message 220. Local terminal 110 then determines whether the internal timer value exceeds second response time-out interval 270 (block 345). For example, 2.2 seconds is a standard second response time-out interval within the telecommunications industry.
When the internal timer value exceeds second response time-out interval 270 (block 345), local terminal 110 assumes that remote terminal 152 is inoperative and local terminal 110 fails the call (block 355), thus terminating the modem training procedure.
When the internal timer value does not exceed second response time-out interval 270 (block 345), local terminal 110 determines whether remote capabilities message 225 has been received (block 350). When remote capabilities message 225 has not been received (block 350), local terminal 110 again determines whether second response time-out interval 270 has been exceeded (block 345). The procedure then iterates as shown in FIG. 3.
When remote capabilities message 225 has been received (block 350), local terminal 110 continues the modem training procedure (block 360) at the negotiated data rate and in accordance with the requirements of the internal modems of local terminal 110 and remote terminal 152.
FIG. 4 is a flow diagram of a prior art protocol for remote terminal modem training and capabilities message exchange. Referring also to FIGS. 1 and 2 and associated text, remote terminal modem training and capabilities message exchange begins (block 410) when remote terminal 152 receives local modem tone 210 (block 415). Remote terminal 152 then waits a required signaling delay time (block 420). For example, a required signaling delay time may be zero seconds (no delay) or one second. After the required signaling delay time has expired (block 420), remote terminal 152 transmits remote modem tone 215 (block 425).
Remote terminal 152 then determines whether local capabilities message 220 has been received (block 440). When local capabilities message 220 has not been received, remote terminal 152 continues to monitor incoming data until local capabilities message 220 is received. When remote terminal 152 receives local capabilities message 220 (block 440), remote terminal 152 transmits remote capabilities message 225 (block 445). Remote terminal 152 then continues the modem training procedure (block 450) at the negotiated data rate and in accordance with the requirements of the internal modems of local terminal 110 and remote terminal 152.
A signal transmitted by a local terminal (e.g., local terminal 110, FIG. 1) through a single satellite hop experiences a time delay before it is received by a remote terminal (e.g., remote terminal 152, FIG. 1). The time delay results from the signal propagation time from the local terminal to the satellite and down to the remote terminal. The delay length depends on the distances between each terminal and the satellite.
Government performance specifications require that STUs operate over two satellite hops (e.g., satellites 131, FIG. 1) on the PSTN (e.g., PSTN 141, FIG. 1) side. Additionally, it is not uncommon to require two or more satellite hops (e.g., satellites 130, FIG. 1) between a terminal (e.g., local terminal 110, FIG. 1) and a PSTN interface (e.g., modem 138, FIG. 1). Therefore, a message may be transmitted through four or more satellite hops before reaching a final destination. Each satellite hop contributes additional time delay to the total message propagation time.
Table 1 summarizes approximate timing delays inherent in the four satellite hop inoperative configuration of conventional communication system 101 depicted in FIG. 1. As used in Table 1, "start" is the element number in FIG. 1 where the delay originates and "end" is the element number in FIG. 1 where the delay ends.
TABLE 1 ______________________________________ CONVENTIONAL COMMUNICATION SYSTEM TIMING DELAYS start end delay explanation delay ______________________________________ 110 114 local tone detect time .5 sec 114 138 two satellite hops .6 sec 138 152 two satellite hops .6 sec 152 138 two satellite hops .6 sec 138 114 two satellite hops .6 sec 114 detect of remote tone .1 sec 114 110 modem processing delay .1 sec total 3.1 sec ______________________________________
The total round trip delay of 3.1 seconds for a system having four satellite hops exceeds the standard second response time-out interval of 2.2 seconds. The total round trip delay is 2.5 seconds with three total satellite hops. Thus, the prior art protocol does not work with three or four satellite hops between local terminal 110 and remote terminal 152.
With only two total satellite hops between local terminal 110 and remote terminal 152, the total round trip delay is 1.9 seconds. Therefore, the prior art protocol does work with two satellite hops. Additional network delays, typically hundreds of milliseconds, are not included in the estimates provided in Table 1. With these additional delays, the prior art protocol may not work with two satellite hops.
A significant drawback of the prior art protocol is that modem training response time-out intervals for a particular communications system may limit the number of satellite hops allowable between the local terminal and the remote terminal to as few as one or two satellite hops. However, for some applications, communications ability through four or more satellite hops is desirable.
Thus, what is needed is a practical, economical method and apparatus allowing successful modem training to occur when more than two satellite hops exist between the local terminal and the remote terminal. What is particularly needed is a modem training method and apparatus allowing two or more radio satellite hops and two or more land-based equipment (e.g., PSTN) satellite hops.